1. Field of the Invention
The present invention relates to a data-transmitting apparatus that transmits, for example, still-picture signals to a large-image display apparatus such as a projector or a PDP. The invention also relates to a data-receiving apparatus that receives signals transmitted and a data-transmitting system that uses the data-transmitting apparatus and the data-receiving apparatus.
2. Description of the Related Art
Image data processed in computers at present has high precision. To cope with this, the image resolution achieved by display elements has increased. In view of this technical trend, the technique of inputting image signals from computers to a liquid crystal projector and projecting the images represented by the signals, thereby to perform presentation has come into wide use in recent years. The cable generally used to input image signals from a computer to a projector is a bundle of five lines, i.e., three image signal lines and two sync signal lines. The image signal lines are R-image signal line, G-image signal line and B-image signal line. The sync signal lines are horizontal-sync signal line and vertical-sync signal line. Due to the band allocated to the signals, the cable is thick and short. This inevitably restricts the position of the computer and that of the projector, ultimately imposing limitations on the representation. To solve this problem, an image-transmitting system that transmits image signals by radio has been proposed (see, for example, Patent Document 1: Jpn. Pat. Appln. Laid-Open Publication 11-004461.)
In this image-transmitting system, input image data is stored in the transmitting-side frame memory. The data is read from the transmitting-side frame memory as serial data and transmitted in the form of infrared rays or radio waves. In the receiving side, the serial signal is converted back to a parallel signal. The parallel signal is stored in the receiving-side frame memory. The parallel signal is read from the receiving-side frame memory. From the parallel signal thus read an RGB-image data signal is restored.
Among the image signals generally used to perform representation is an image signal of XGA format. In the XGA format, the image size is 1024 pixels (in horizontal direction)×768 pixels (in vertical direction) and the clock frequency is 65 MHz if the frame frequency is 60 Hz. Each pixel requires 8 bits to achieve RGB presentation. Hence, the transmission rate for serial signals is about 1.6 GHz. For example, the signals may be sent by spatial transmission in infrared-ray communication at a band of about 100 MHz. In this case, the signals can be transmitted during a plurality of frame periods. However, only one frame of image can be transmitted during 16 frame periods. Here arises the problem that the cursor in the image moves unnaturally when a presenter moves a mouse.
The ordinary-image transmitting system employs an image-compression/coding scheme such as MPEG (Moving Picture Experts Group) or JPEP (Joint Photographic Expert Group). Whichever scheme the system employs, the quality of images is important. MPEG and JPEP are image-compression/coding schemes appropriate for encoding, mainly, natural images (video data). They are not fit for encoding computer outputs (particularly, high-precision text images). Further MPEG encoding involves motion detection for acquiring a motion vector from image data and also discrete cosine transform (DCT). The circuit that performs MPEG encoding inevitably be a large one. As a consequence, the image-transmitting system will be a large-scale and high-cost system.
In the case where an output from a computer is a file of the image displayed, the file can be transmitted via a LAN (Local Area Network). Recently a system using a radio LANs using 2.4 GHz band has come into being. This system can preserve the quality of the original images, but is disadvantageous in the following respect. Assume that, in the system, a computer is the transmission side and a display (monitor or projector) is the receiving side. Then, there is no problem with the transmission side, but the display, i.e., receiving side, needs to have a LAN function. Without a LAN function, the receiving side must have a computer. Consequently, the cost of the system will increase. Additionally, the LAN connection cannot be utilized only if a VGA cable connects the computer and the display. The method of setting the system is inevitably complicated.
In view of the above, a signal-transmitting system has been proposed. This system requires no complicated setting procedures. The system uses a method of reliably reproducing image data, regardless of the positions of the transmitting side and receiving side or the transmission rate. It is determined whether the image data of the frame now input differs from the image data of the previous frame input. Subsequently, a region of the image data, which is to be transmitted, is set, and the data showing the difference between the frames compared is added to the image data representing the region. The resultant image data is transmitted. (See, for example, Patent Document 2: Jpn. Pat. Appln. Laid-Open Publication No. 2001-103491.)
However, a greater part of the original image data input may change, or the data of an image to be displayed may greatly change as in the case of a page change. In either case, the amount of data to be transmitted increases, inevitably causing a time lag between the start of image-data transmission and the displaying of the image. Thus, the response time, i.e., time elapsing until the image is displayed, differs, from image data to image data. As a consequence, the image cannot be displayed at the very time it is explained in a presentation. This makes it impossible to perform smooth presentations.